There is only one file type that uses the .OTT extension:
OpenDocument Document Template
An OTT file is a document template saved in OASIS' OpenDocument standard format and created by word processors included in the OpenOffice and LibreOffice suites. It contains the page layout and default styles for a text document. OTT files are used for authoring .ODT documents that have the same appearance and formatting.
Updated: February 14, 2017